THE EVOLUTION OF THE PEIMATES 321
were derived. The remains of the actual ancestral placentalia have not been discovered, so it is necessary to make a hypothetical type; and deductions from comparative anatomy and palaeontology have led observers to place the hypothetical animals in the Cretaceous Period.
It is believed that the ancestral, placental Mammals were small creatures with comparatively simple teeth, arranged according to the formula I| C^PMJMf. Their food habits were insectivorous, and they may have been arboreal. They may be regarded as forming a central reservoir from which streams of life flowed in different directions; and each stream or radiation ended in one or more of the modern Mammalian Orders
One branch or radiation is represented to-day by the Tree Shrews (Tupaiidse), which are in many ways very primitive. A second branch has as its sole living representative the Flying Lemur (GaleopitJiecus volans), which is allied to the Insectivora and Cheiroptera.
Of the remaining branches of the ancestral stock the Primates alone concern us here.
The earliest remains of undoubted Primates have been found in the Eocene rocks of North America. And as these are remains of bothLemuroidea and Tarsioidea, which represent different lines of evolution, we must assume that they arose from a common ancestor, which lived at an earlier period. It is now believed that the separation of the Lemuroidea and Tarsioidea from common ancestral Primates took place in the Paleocene, or even as far back as the Cretaceous Period. m
The ancestral Primates separated from the other 21